Move RequestReplyHeaderMapper to core

This commit moves the RequestReplyHeaderMapper and related base
implementation from Spring Integration to the core messaging module.

To prepare the migration of AMQP mappers to the spring-amqp module
itself, the priority header value is also now part of the standard
headers.
We should either remove these two, or allow subclasses to somehow override the default behavior. In Spring Integration, we now permit these headers to be mapped (if they hold a `String` instead of the default `MessageChannel`).